Electrodeposition as a coating application method involves deposition of a film-forming composition onto a conductive substrate under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because, by comparison with non-electrophoretic coating means, electrodeposition offers increased paint utilization, improved corrosion protection and low environmental contamination.
Initially, electrodeposition was conducted with the workpiece being coated serving as the anode. This was familiarly referred to as anionic electrodeposition. However, in 1972, cationic electrodeposition was introduced commercially. Since that time, cationic electrodeposition has steadily gained in popularity and today is by far the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition.
Although surface coatings of excellent quality can be achieved by means of cationic electrodeposition, a problem associated with this means of coating is the development of surface defects upon curing, particularly craters. The cause of such defects can be a result of the very nature of the electrocoating composition components, that is, causes inherent in the system. Typically, however, impurities which are carried into the electrocoating bath with the workpiece cause such surface defects. Examples of such impurities include lubricating oil, anti-corrosion grease, joint sealing compound and the like. As the electrocoating composition is deposited onto the conductive substrate, the impurities are carried along with the coating composition and are deposited as well. When the coated substrate is cured, craters are formed due to the incompatibility between the impurity and the resinous phase of the electrocoating composition.
Due to their low cost and commercial availability, the use of minerals as rheology modifiers and/or fillers in coating compositions is common. Traditionally these have included such minerals as wollastonite, attapulgite, kaolin, talc, mica and calcium carbonate. These materials have been incorporated in both treated and untreated forms, the most common treatment being silanization, to compatibilize the mineral hydrophilic surface with the host polymer. Minerals such as mica and talc, which are layered silicate materials, have a platy morphology with aspect ratios (aspect ratio=ratio of particle width to particle thickness) of typically less than 50. These materials are commonly known for use in coatings as rheology modifiers and fillers and, in addition, due to parallel interlamination of the silicate layers, can improve barrier properties.
To effectuate adequate crater control in electrocoating compositions using these common clay minerals, a high level is needed, e.g., 2 to 20 percent by weight based on total solids of the electrodeposition bath. At these levels other liquid coating properties, for example, working viscosity and stability, and cured coating properties, such as appearance, can be adversely affected. Also, clays generally known for use in coatings are typically not desirable for use in electrocoating compositions because ionic contaminants contained within these materials tend to destabilize the electrodeposition baths and cause settling.
Also known in the art is the preparation of aqueous dispersions of exfoliated layered materials, such as vermiculite and bentonite clays. Generally, the clay particles are contacted with an ionic solution to effect cation exchange within the interlayer spacing, thereby permitting swelling of the spacing to bring about essentially complete delamination ("exfoliation") upon immersion in aqueous media. These ionic solutions typically contain lithium, alkyl ammonium and/or ammonium carboxylic acid cations. The exfoliated particles are then dispersed under high shear. These dispersions can be used to form or cast films by using drawdown or spraying techniques followed by evaporation of the aqueous phase.
Additionally, it is known to use smectite minerals, particularly montmorillonite clays, in plastic composite materials. These materials are a family of clays having a 2:1 layer structure and, typically, aspect ratios ranging from about 200 to 2,000, which are orders of magnitude greater than for conventional fillers such as mica and talc. Exfoliation of these materials is accomplished using a two-step process. The clays are treated with polymers which contain functional groups, e.g., hydroxyl, amine, and amide groups, to enlarge the interlayer spacing such that insertion and ionic attachment of organic molecules to the platelet surfaces can occur, a process known as "intercalation" with the product formed thereby being known as an "intercalate". During a subsequent polymerization/compounding step in composite formation, individual platelets delaminate or "exfoliate" and are embedded throughout the polymer matrix.
Such polymer-clay composites are described in U.S. Pat. No. 5,853,886 wherein a proton-exchanged layered silicate is intercalated with a basic group-containing polymerizing component. The intercalate is then contacted with a thermoset or a thermoplastic resin system which reacts with the polymerizing component thereby exfoliating the intercalate and forming a hybrid polymer-clay composite.
Although the prior art discloses dispersions of intercalated layered silicate materials in aqueous media, acidified aqueous stable dispersions of silicate materials derived from layered silicate materials which have been exfoliated with a cationic group-containing polymer are not disclosed. Likewise, although the art teaches the use of the above-described dispersions to form films of exfoliated layered silicate materials, the prior art does not disclose the use of such dispersions in liquid coating compositions, particularly electrocoating compositions.
Moreover, although the art teaches the intercalation of layered silicate materials with a polymer containing functional groups to enlarge the interlayer spacing, with subsequent exfoliation in the presence of a host polymer to form platelet/polymer composite materials, there is no teaching in the prior art of the use of silicate materials derived from a layered silicate material which has been exfoliated with a cationic group-containing polymer or a polymer which contains functional groups which can be post-reacted to form cationic groups, in coating compositions, particularly in electrodepositable coating compositions.
It has been found that the inclusion in electrocoating compositions of acidified aqueous dispersions of silicate materials, such as montmorillonite clays, which have been exfoliated with a cationic group-containing polymer or a polymer which contains functional groups which can be post-reacted to form cationic groups, provides improved crater resistance. Moreover, it has been found that these silicate materials effectuate improved crater resistance at levels much lower than those levels needed for more conventional clay materials and, therefore, liquid coating properties, such as electrodeposition bath stability and working viscosity, and cured coating properties, such as adhesion and appearance are not adversely affected.